The acquisition of a spread-spectrum code signal, also called CDMA (Code Division Multiple Access) signal, requires finding the frequency deviation (between the received CDMA signal and the local frequency signal used in the receiver for downconversion), the code phase offset (between the received CMDA signal and the local CDMA signal used for despreading the received CDMA signal), and the transmitted CDMA code. Signal acquisition is required in satellite positioning systems such as, for example, Galileo and GPS (Global Positioning System), in satellite-based CDMA communication systems and in terrestrial cellular communication systems such as, for example, UMTS (Universal Mobile Telecommunications System), CMDA-2000, TD-SCDMA (Time Division Synchronous CDMA) and IS-95 (Interim Standard 95).
A CDMA transmitter modulates the CDMA signal to be sent out onto a sinusoidal carrier in the radio-frequency band (RF) and radiates it via an antenna. The CDMA receiver shifts the RF CDMA signal received via an antenna into the baseband. For this so-called downconversion of the received CDMA signal, one or more local reference oscillators are used which are implemented in the receiver. To be able to downconvert the received CDMA signal precisely into the baseband, the CMDA receiver requires knowledge about the frequency of the received CDMA signal and knowledge about the frequency of the local reference oscillator or oscillators used. Due to a number of error sources, neither the received frequency nor the frequency of the local reference oscillator or oscillators is accurately known. The received frequency of the incoming CDMA signal is changed by the Doppler frequency shift which is influenced by a movement of the CDMA transmitter (e.g. satellite) and by a movement of the CDMA receiver. The frequency of the reference oscillator or oscillators is only known within a particular range of tolerance and shifts due to temperature influences, voltage changes, radiation influences, vibrations and other physical effects.
Large Doppler shifts occur particularly in satellite positioning systems due to the satellite movement. It has already been proposed to facilitate acquisition for the receiver by signalling the Doppler shift to be expected. However, the expenditure required for the signalling and too high an inaccuracy of the Doppler frequency shift data transmitted are disadvantageous.
To determine the frequency deviation during signal acquisition, various procedures are known. The possible frequency range is typically subdivided into frequency sub-ranges (so-called frequency bins). The search in the frequency domain is then performed by testing the individual frequency bins. For this purpose, the CDMA receiver downconverts the received signal with the centre frequency of a first frequency bin and uses the down-converted signal as test signal for synchronization. If no synchronization between the CDMA transmitter and the CDMA receiver can be achieved with the test signal, the centre frequency of the next frequency bin is selected and the process is repeated. This method is continued until the correct frequency bin is found with which synchronization is successful. The centre frequency of this correct frequency bin is the result of the frequency search in this known acquisition method.
The disadvantageous factor in this method is that the residual error of the method is only determined by the width of the frequency bin. Furthermore, the sensitivity of the method is influenced by the width of the frequency bin. To achieve a low residual error and high sensitivity, the frequency width of the frequency bin must be typically selected to be small. This results in a relatively long acquisition period, the period needed for the acquisition process increasing linearly with the residual-error accuracy needed.